I would second Magnussen on those wool pants for any kind of cold weather work.
Those are the exact pants I wore when researching/tracking carnivores through the winter as a USFS Biologist for two consecutive years in the Oregon and Washington Cascades. Those and my heavy 16 corked pack boots kept me warm and toasty, even when I was running long-track snowmachines.
When backpacking, I generally only wear nylon shorts, rarely I wear Patagonia zip offs (GI IIs I believe). I always zip off the legs and never put them back on, so dont really see the point. When traveling long distances (out of state/country) with my family I often take/wear the zip offs, due to the versatility.
During the cooler part of my field season (feb, mar, ~apr, ~sep, oct, nov) I wear poly/cotton BDUs from two sources: BDU.com or CombatCloth (usually from Brigade Quartermaster). I find they work very well for me and the type of work I do. They have a professional look for the public (have received several compliments on them, for what it is worth), but last several years due the quality of construction and material. I find they are very comfortable, have plenty of carry space for all the junk I often end up needing to carry.
I used to wear Carhart double knee pants, but just found them too restrictive for fieldwork, for me. And they just held water too easy. They are tough and last, though. They are regular wear for me outside of field work.
During the warmer part of my field season, I usually just wear cotton shorts.
I would really like to try out the 5.11 pants; several folks have suggested those to me. Perhaps I will pick up a pair or two this year.
I hadnt seen those Taclite pants gruntinhusaybah likes. I will take a closer look at those, too.
Good luck and let us know what you decide and how you like them.
Brome